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Śrī Īśopaniṣad
The material bodies of all animals and men are foreign to the living entity, and change according to the desire of the being for sense gratification. In this cycle of evolution one changes bodies one after another: from aquatic life—when the world was full of water—to vegetable life, from vegetable life to worm life, from worm life to bird life, from bird life to animal life, and from animal life to the human form.
The highest development of life is this human form when it is possessed of a full sense of spiritual knowledge, and the highest development of spiritual sense is described in this  mantra  of the  Veda : one should give up this material body to be turned into ashes, and allow the living air to merge into the eternal reservoir of air. The living being's activities are performed within the body by movements of different kinds of air, and this is called, in sum, the  prāṇa-vāyu . The  yogis  generally study to control the airs of the body, and the soul is supposed to rise up from one circle of air to another till it rises onto the  brahma-randhra , or highest circle of air. Then the perfect  yogi  can transfer himself to any planet he likes. The process is to give up one material body and then enter into another body, and the highest perfection of such a bodily change is possible when the living entity is able to give up this material body altogether, as is suggested in this  mantra . He may then enter into the spiritual atmosphere, where he develops a completely different quality of body—a spiritual body—which never has to meet death or change.
Here in the material world one has to change his body, forced by material Nature on account of his different desires for sense gratification. These desires are represented in the various species of life, from the germs to the most perfect material bodies, those of Brahmā and the demigods. All of these have bodies of matter in different shapes, and the intelligent person sees oneness not in the variety of bodies, but in the spiritual identity.
The spiritual spark which is the part and parcel of the Supreme Lord is the same either in the body of a hog or in the